Parameters important for the suitability of a film for the packaging of perishable goods are the oxygen barrier and also, in the case of transparent packaging, the permeability of the film to UV (the ultraviolet portion of light). The second item is particularly important in the case of foods, for example those presented in supermarket counters which are subject to irradiation by lamps with relatively high UV emission. If the UV permeability of the packaging here is too high the result can be unattractive discoloration and changes in taste.
A particularly important third criterion for the suitability of a packaging film is its price, since in the end-user market even relatively small price differences can shift the purchasing decision in the direction of a more favored product, even if the packaging does not meet high quality requirements with regard to shelf life.
The polyester-based packaging films currently available in the market are almost exclusively PET (polyethylene terephthalate)-based films, such as Hostaphan® RNK. Although these films are low-priced and generally have an acceptable oxygen barrier for many applications, this barrier is nevertheless often inadequate, in particular for transparent, unmetallized packaging. In addition, PET retains relatively high permeability to UV light.
Films made from PEN (polyethylene naphthalate) provide a substantially higher oxygen barrier and also have lower UV permeability, but are uneconomic due to the high price of PEN and are therefore little used in industry.
Although this problem can be reduced to some extent by using multilayer films which comprise one or more PEN layers on a PET base (e.g. DE-A-197 20 505 or WO 01/96114), these films have problems with delamination of the layers made from the different polyesters and are relatively difficult to produce. In addition, the regrind arising cannot subsequently be reused for a product of the same value, since PET and PEN become mixed during the regrinding process and can therefore no longer be utilized to produce the high-value layers comprising 100%, or almost 100% by weight, of PEN. The material can only replace the comparatively inexpensive PET in other layers or other films. For these reasons, although these films are less expensive than films composed mainly of PEN, they are considerably more expensive than standard PET packaging films and are therefore used only in niche markets with particular requirements.
Films made from blends of PET and PEN have also been described (e.g. GB 2 344 596), but there is no indication here as to how these blends can be used to produce a film with improved UV barrier and oxygen barrier, capable of being used to produce transparent packaging with increased shelf life. Nor is there any indication of the quantitative PET/PEN proportions suitable for producing films with a good oxygen barrier and a good UV barrier, and there is no indication as to how the pinhole problem may be solved.
Another very general way of improving the UV permeability of polyester films is the addition of UV stabilizers/absorbers, e.g. Tinuvin® from Ciba Specialty Chemicals (Switzerland). However, the use of these compounds is likewise associated with relatively high costs and, depending on the stabilizer used, also creates problems of suitability for direct food contact.
Prior-art films often have what are known as pinholes—holes or thin spots. These pinholes arise inter alia when large, loosely bonded particles fall out from the film, or when adjacent film layers cause damage due to large particles present in the material, or when there are particles on rollers. These pinholes markedly reduce the oxygen barrier provided by the packaging in which they are present, and can thus lead to spoilage of the goods present in the pinholed packaging. This applies in particular to the relatively long storage times intended to be achieved with the aid of a film meeting the object of the invention. Although films available in the market generally also have a low total number of pinholes, i.e. up to 30 per 100 m2, depending on the size of the packaging this can nevertheless be a cause of spoilage of some percentage of the packed goods.
An object of the present invention was then to provide a film which provides a better oxygen barrier and UV barrier than straight PET films and which has no, or only a very small number of, pinholes, and which can be produced at low cost, and which is suitable for packaging applications, and which, when compared with packaging comprising PET films, ensures an improved lifetime for the packed goods, even when UV doses are relatively high.